The present oil extracting rod ground screw pump driving system employs common asynchronous motor through belt transmission to drive gear box, and the gear box then drives the polished rod of the screw pump to make rotary motion. The dynamic system for such a screw pump has the following disadvantages:
1. The transmitted power is limited. When the screw pump requires bigger torque (for example, a torque greater than 1500 Nm) or the system transmitted power is larger (for example, greater than 10 kW), the belt may easily slip and be damaged, the teeth of the gear box are easily broken and damaged, making it difficult to satisfy the requirement.
2. A large amount of mechanical maintenance and high operation cost are needed. Rotary sealing parts must be installed within the gear box because there is lubricating oil in the gear box. In practical operation, problems with lubricating oil leakage occur in the gear box. Moreover, the reliability of the belt transmission is poor, which leads to a large amount of mechanical maintenance and high operation cost during practical operation.
3. The efficiency is low. In general, for the convenience of starting, the equipped motor is always chosen to have 3 times the power as that of the practical operation power, which leads to the operation of the asynchronous motor at a low load rate, thus the efficiency is rather low and is generally about 75%.
4. The regulation of parameter is not convenient. When the speed needs to be re-regulated if the downhole working conditions vary, generally, the belt pulley must be changed or a variable pole adjustable speed motor must be employed. Changing the belt pulley is very troublesome, while just one to two gears can be used through the speed adjustment of the variable pole adjustable speed motor, which makes it inconvenient in practical usage.
5. The noise is great. The noise during the operation of mechanical speed reducing devices and high speed asynchronous motor is great and thus it is not suitable to installed in residential districts.
6. The occupied area for the equipment is large. Since the belt pulley system requires a large occupied area, the equipment is not suitable for situations that require a small area, such as an offshore oil-production platform, cluster well, etc.
A low speed motor direct drive screw pump device has been provided in Chinese patent publication no. CN 1683791 entitled “Directly driving screw pump device”. The device consists of polished rod, a shaft coupling, a low speed motor, a motor controller, a fixing seat, and a screw pump. The motor shaft of the low speed motor is hollow and positioned within the polished rod. The motor shaft is fixed with to the polished rod using a shaft coupling. The polished rod passes through the central hole of the fixed seat and connects to the shaft of screw pump downhole. The low speed motor is fixed onto the fixed seat. The motor controller is installed within the terminal box of the low speed motor, and a matching controller for the low speed motor is employed. Such a motor directly driving a screw pump does not use a belt transmission or a speed change gear device. As motor shaft is connected to the pump shaft of screw pump through polished rod, the reactive power loss is decreased, the system efficiency and reliability of screw pump is increased, the oil extracting cost is reduced, and at the same time, the volume and weight of the screw pump is reduced.
However, the device does not allow precise regulation of any value of motor speed from zero to maximum by the controller, and the device has the disadvantage of difficult speed regulation, and the efficiency of the system must be increased.